This invention relates to temperature measurement of an object in a radiative environment, such as a semiconductor wafer in a rapid thermal processing (RTP) chamber, by computation of the temperature from noncontact in-situ measurement of emissivity and radiance of the object. As a particularly advantageous feature, the emissivity is determined in situ and in real-time by measuring wafer reflectance without an external light source through the use of a chopper having regions of different transmissivity and reflectance. The ratio of light intensities detected through the open and reflecting portions of the shutter is used to determine the emissivity of the wafer for pyrometric temperature measurement purposes. Furthermore, in situ, real-time radiance measurements are obtained through the open region of the shutter. Interference due to radiation from the heating lamps is excluded by appropriate wavelength selection and by the use of a light pipe that collects light from a defined direction. This technique enables single-point measurements of the temperature of a hot object in a radiative environment, and it can be used for precise control of temperature in this environment. Through the use of several sensors at separate locations across the wafer along with appropriate control of the lamp intensities in different zones of the reactor, it is possible to control the temperature profile (uniformity) across the wafer during the initial heating ramp as well as at steady state. Since the temperature measurement is attained in real time, it can be used advantageously to control both the heating of the chamber and the temperature of the heated object.
In manufacturing processes, the heating of a workpiece is employed often in one or more of the manufacturing steps in order to obtain a desired end product. This is particularly true in the case of manufacture of semiconductor circuits and other semiconductor products wherein a wafer is treated by various steps of etching, doping and cleaning to construct a complex configuration of circuit components on the wafer. During various stages of the manufacture, the temperature of the wafer must be elevated, for example, to temperatures in the range of 600-1200 degrees Centigrade. It is the practice to place the wafers in a furnace to obtain the desired temperature; however, more recently the use of RTP apparatus has come into use as a more attractive alternative to furnace processing of wafers because RTP provides for a rapid, single wafer processing with advantages of better control and lower manufacturing costs. However, especially in view of the short duration of many RTP steps, it becomes increasingly important to measure and to control the temperature closely. Control of temperature requires a measurement of the temperature, and it is advantageous to employ in-situ temperature measurement by a viewing of radiation of the wafer, or other workpiece, without physically contacting the wafer. Such a temperature measurement may be done at several discrete points on a wafer to obtain data of any nonuniformity in temperature, or temperature profile, across the wafer.
Accurate measurement of temperature is obtained from a combination of both emissivity data and radiance data of a workpiece or other object in a radiative environment wherein the data is obtained at a specific wavelength, and the temperature is calculated from this data. An improvement in the measurement of emissivity directly benefits the temperature measurement and allows for a more accurate determination of the temperature.
Typically, emissivity of an object has been determined from a measure of reflectance of the object. In the past, such measurement has employed directional reflectance using a filtered lamp radiation or a laser as a source of light at an appropriate wavelength. Such techniques are taught in the following U.S. patents: U.S. Pat. No. 4,647,774 (Quantum Logic Corp., Mar. 3, 1987), U.S. Pat. No. 4,956,538 (Texas Instruments, Inc., Set. 11, 1990), U.S. Pat. No. 5,156,461 (Texas Instruments, Inc., Oct. 20, 1992), and U.S. Pat. No. 4,919,542 (A.G. Processing Technology, Apr, 24, 1990). The light, typically infrared light, is directed as an incident beam at the object from a point external to the radiative environment of the object. Light from the incident beam is reflected from the object as a reflected beam. In a situation of major concern herein, the object is a semiconductor wafer, and the radiative environment is an RTP chamber enclosing the wafer. The intensities of the incident and the reflected beams in a given direction are used to derive the wafer reflectivity at the desired wavelength. The fraction of the reflected light collected in the chosen direction is also measured using scattering measurements described in U.S. Pat. No. 5,156,461 (Texas Instruments, Inc.) The measured bidirectional reflectance in combination with the specularity measurements, yield information about the total reflectivity of the wafer. A disadvantage of this method is the need for additional hardware in the form of an external light source which increases the cost and size of the temperature sensing system.
Other apparatus employed in the measurement of emissivity includes the use of two fibers mounted in an RTP chamber in such a fashion that one of the fibers views the output of lamps used to heat the wafer, and a second of the fibers views radiation coming from the wafer, the latter including thermal wafer emissions as well as the lamp radiation reflected from the wafer. This technique is described in U.S. Pat. No. 5,154,512 (Luxtron Corp.). Typically, the measurements are made at 0.9-1.5 micron wavelength where the lamp intensity is high and the wafer is opaque. This method makes use of the fact that the intensity of the lamp emission, as well as the reflected lamp radiation, has an AC component due to imperfect electrical filtering in the supplying of current to the lamp, while the wafer thermal emission is continuous because of its long thermal time constant. The reflectance is then measured as the ratio of the AC components of the light from the two fibers. Emissivity is then calculated from Kirchoff's Law. In this method, since the light is collected from a fairly large region on the wafer, the signal of the reflected radiation is not extremely sensitive to surface roughness. A disadvantage of this method includes the need to modify the housing of the RTP apparatus to allow the sensors to be installed at appropriate locations.
There are several well-known and severe problems when emissivity is not measured during the RTP process due to a variation in the emissivity of the object. Such variation degrades the accuracy and repeatability of pyrometric temperature measurements by introducing errors as much as several tens of degrees Centigrade. In general, the wafer emissivity is not constant from wafer to wafer and for a given wafer during a manufacturing process because the emissivity is a function of a number of variables including wafer temperature, surface film thicknesses, films on the heating chamber wall which alter the chamber reflectivity, backside roughness of the wafer, and process history. Therefore, it is necessary to monitor the emissivity of wafers in situ and in real time, and in a non-contact manner to insure correct temperature measurement. The use of other forms of measurements wherein the object must be removed from its environment, or by use of monitor wafers to provide a substitutional form of measurement, or the use of theoretical calculations to estimate emissivity do not provide the desired amount of accuracy because of variations in the manufacturing process and in the wafer itself. It is known that, by use of Kirchoff's Law, emissivity can be determined solely from reflectance measurements for the case wherein the wafer is opaque, including the situation wherein the opacity is induced by temperature or by doping. For highly doped wafers, or for measurement of wavelength less than approximately one micron, emissivity measurement from reflectance alone is possible for objects at room temperature and above. However, for emissivity measurements conducted at longer wavelengths or for lower doping, such as at a wavelength of three microns for an undoped silicon wafer, and in the absence of films on the wafer, determination of emissivity from reflectance measurements alone can be accomplished only at temperatures higher than approximately 700 degrees Centigrade wherein the wafer is opaque.
A problem arises in that presently available apparatus and procedures are not readily implemented for standard forms of RTP apparatus without extensive modification and/or introduction of ancillary hardware to the RTP apparatus, this creating a need for measurement equipment and procedure which are easier to implement.